Borehole acoustic logging-while-drilling (LWD) for formation evaluation has become an indispensable part of hydrocarbon reservoir assessment (Tang et al., 2002; Cittá et al., 2004; Esmersoy et al., 2005). However, the detection of acoustic formation arrivals1over tool mode contamination has been a challenging problem in acoustic LWD technology. This is because the tool mode contamination in LWD is more severe than in wireline tools in most geological environments (Tang et al., 2002; Huang, 2003).
In this paper we propose a new method for separating tool waves from formation acoustic waves in acoustic LWD. This method is to measure the seismoelectric 2signal excited by the LWD acoustic waves.
The acoustic waves propagating along the borehole or in the formation can induce electric fields. The generated electric field is localized around the wave pulses and carried along the borehole at the formation acoustic wave velocity. The LWD tool waves which propagate along the rigid tool rim can not excite any electric signal. This is due to the effectively grounding of the drill string during the LWD process makes it impossible to accumulate any
excess charge at the conductive tool – borehole fluid interface. Therefore, there should be no contribution by the tool modes to the recorded seismoelectric signals.
In this study, we designed the laboratory experiments to collect simulated LWD monopole and dipole acoustic and seismoelectric signals in a borehole in sandstone. By analyzing the acoustic and electric signals, we can observe the difference between them, which are the mainly tool modes and noise.
Then we calculate the similarity of the two signals to pick out the common components of the acoustic and seismoelectric signals, which are the pure formation modes. Using the seismoelectric signals as reference, we could filter out the tool modes. The method works well.
To theoretically understand the seismoelectric conversion in the LWD geometry, we also calculate the synthetic waveforms for the multipole LWD seismoelectric signals based on Pride’s theory (Pride, 1994). The synthetic waveforms for the electric field induced by the LWD-acoustic-wave along the borehole wall demonstrate the absence of the tool mode, which is consistent with the conclusions we get in the experimental study.